Hydrogen-based energy storage systems are known and have been investigated as solutions for powering many technological developments including, in particular, electric vehicles. Hydrogen-based energy is particularly desirable because hydrogen gas reacts cleanly with air in a fuel cell, producing water as a by-product. Such systems face challenges that have not, as of yet, found a suitable solution. Hydrogen-based solid-state energy storage systems also lose significant capacity as they operate through charging and discharging cycles. It would also be advantageous to improve the amount of hydrogen that can be stored and discharged from the same weight or volume of storage material. This would promote the use of hydrogen fuel cells in light-duty vehicles and other applications.
Complex metal hydrides show potential for hydrogen storage applications due to their high hydrogen content. In particular, bulk compositions of metal imides (—NH) and amides (—NH2) consisting of Li, Mg, Na, B, Al, Be, Zn and their mixtures have shown potential as reversible hydrogen storage materials (reversible meaning rechargeable by a reverse reaction after hydrogen release). These metal hydride mixtures are typically synthesized by ball milling. In the hydrogenated state (outside of extreme operating conditions), such compositions comprise a metal amide and a metal hydride and in the dehydrogenated state, the materials of such compositions comprise a metal imide and a metal hydride Bulk metal amides have several drawbacks including limited cycle-life, occurrence of stable nonreactive imide species, and contamination of hydrogen with ammonia gas. Ammonia release irreversibly damages the hydrogen storage material since it removes nitrogen from the material, leading to reduced capacity. In addition, bulk metal-nitrides form as the end products and are known to be reluctant towards hydrogenation, adversely affecting the kinetics of the hydrogenation reaction needed for a fully cycling material.
Nanoconfinement has been explored as a promising route to improve the LiNH2/LiH energy storage systems. Janot and co-workers developed an innovative synthetic route to Li3N@carbon (the @ sign meaning “confinement in”) by wet impregnation of mesoporous carbons using solutions of lithium azide, followed by a thermal treatment allowing the transformation of lithium azide into lithium nitride. The resulting Li3N@carbon composites displayed improved hydrogen storage properties compared to bulk LiNH2/LiH with fast hydrogen absorption/desorption kinetics at 200° C. and above. The 20 wt. % Li3N-loaded composite lead to a reversible hydrogen storage capacity of 1.8 wt. %. (R. Demir-Cakan, W. S. Tang, A. Darwiche and R. Janot, Energy & Environmental Science, 2011, 4, 3625-3631.) Xia et al. expanded this approach by synthesizing carbon-coated Li3N nanofibers by a single-nozzle electrospinning technique. (G. Xia, D. Li, X. Chen, Y. Tan, Z. Tang, Z. Guo, H. Liu, Z. Liu and X. Yu, Advanced Materials, 2013, 25, 6238-6244.) Xia et al, used a polymer-LiN3 mixture as the precursor, which can serve as the templating agent for lithium nitride formation. The carbon-coated Li3N porous nanofibers exhibited some reversibility over 10 de-/re-hydrogenation cycles at 250° C., however there was a significant capacity loss, presumably due to ammonia formation.